1. Field of the Invention
This invention relates to the field of control systems and, more specifically, to settling criterion used in control systems such as for use in disc drive systems.
2. Description of Related Art
The trend in system design is toward faster response times and increased robustness to system variations, for example, resonance. Resonance is a phenomenon that occurs in different types of systems, including mechanical, electrical, and acoustical systems. Many systems, for example, disc drive systems, become unstable in the presence of resonance.
A disc drive system typically consists of one or more discs and control mechanisms for storing data on the discs. Data is stored within concentric tracks on a disc. The reading and writing of data is typically accomplished with a head flown over the disc surface on a thin air bearing. The head is positioned on a desired track using an actuator connected by a support arm assembly to the head. The actuator moves the head in a radial direction to the desired track where it is maintained over the track centerline during both read and write operations. A motor is used to rotate the discs to position the head at a particular location along the desired track.
The movement of the head to the desired track is referred to as track seeking. After a track seek operation, the motion of the head must settle before a read or write operation may be performed. Once the head has settled, it must be maintained over the track""s centerline, referred to as track following. A servo control system is used to maintain the head precisely over the track centerline after a track seek is performed. This is accomplished by using prerecorded position information (servo information) either on a dedicated servo disc or on sectors interspersed between data on the data disc.
The servo information is sensed by a head and demodulated to generate a position error signal (PES). The PES indicates the position of the head relative to the target track centerline. During seeking, the PES is used to maintain desired head velocity and positioning, and to determine when a head has settled according to a settling criterion. During settling, disturbances in the system may cause the head to oscillate around the track centerline. These disturbances may significantly contribute to the PES such that the system must wait for a period of time until the head is sufficiently settled in order to perform a read or write operation. A settling criterion is a manner of checking that the PES is ensured to be within a predetermined threshold window (e.g., the head has sufficiently settled over the track centerline) that enables a read or write operation to be performed.
FIG. 1 illustrates one prior method for determining the settling criterion for a servo system. The settling criterion is based on the pole locations of the dominant complex pole pair of the closed loop transfer function for the servo system. With prior settling criterion, during a read operation, a head is determined to be settled when the position error signal is continuously within a read threshold for at least one half period of the oscillatory frequency of the dominant pole of the transfer function. For example, if the closed loop transfer function has an oscillatory frequency of 1 KHz, the head is determined to be settled when the PES is within the read threshold for a period of 0.5 milliseconds or more. The industry trend, however, is toward decreasing data access times by faster positioning of head assemblies over data locations (i.e., decreasing seek and settle times). With average seek times typically under 10 ms, the settling time period becomes significant. As such, prior settling criterion may significantly contribute to high data access times.
Another problem with such a settling criterion is that the criterion assumes that disturbances are relatively small compared to the system transfer function and thus, can be neglected. Disturbance resonance, for example, generates bias forces that tend to move the head away from the centerline of the track. These forces may be caused by such things as the movement of a flex cable in the actuator connecting the head to the read/write electronics in the disc drive. The movement of the flex cable causes head oscillations at the end of a seek movement, referred to as seek-pumped-resonance. The resonance frequency may typically range from 100 Hz to 180 Hz. In actual operations; the disturbances may be significant compared with system transfer functions and cannot be neglected.
The present invention pertains to a method and apparatus for settling determination in a disc drive control system. The method includes calculating a settling criterion for a parameter using one or more data values. The method may also include determining when the parameter remains within a predetermined limit. The determination may be performed on the data values at a single time point using the settling criterion.
The apparatus includes an actuator in a disc drive system including a head coupled to the actuator to generate a head position signal and a servo controller to receive the head position signal to determine when the head position signal remains within a threshold. The settling criterion could either be an elliptical settling criterion in one embodiment, or a triangular settling criterion in another embodiment.
Additional features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.